The present invention relates to a pressure-sensitive recording paper, and particularly relates to a pressure-sensitive recording paper which exhibits excellent properties even under severe environmental conditions such as high ambient temperature and high humidity or low ambient temperature.
Generally, pressure-sensitive recording paper is composed of (1) a sheet of paper having its back side coated with microcapsules formed by a solution of a colorless electron-donating substance (hereinafter referred to as a color former) having a color-forming reactivity in a solvent (hereinafter referred to as a CB paper) combined with another sheet of paper having its front side coated with a color-developing substance (hereinafter referred to as a developer) which can form a coloring product in reacting with the color former (hereinafter referred to as a CF paper; (2) a sheet of paper having its both sides coated with the microcapsules and the developer, respectively (hereinafter referred to as a CFB paper) combined with the CB paper and the CF paper; or (3) a sheet of paper having its one side coated together with the microcapsules and the developer. In either case of these pressure-sensitive recording papers, an artificial application of a pressure on the paper breaks the microcapsules at the pressured part to bring the color former into contact with the developer resulting in color development.
In the pressure-sensitive recording paper constituted as mentioned above, the matters which give an important influence on the quality of the pressure-sensitive recording paper are the solvent of the color former included in the microcapsules and the wall material which forms the microcapsule. Hitherto, the specific properties required for the solvent have been as follows:
(1) it dissolves the color former to a high concentration,
(2) when applied in the pressure-sensitive recording paper, the velocity of color-development, the color-density and the color stability after color-development are high,
(3) it is stable against light, heat and chemicals,
(4) it is substantially odorless,
(5) it is harmless to human body,
(6) it has a sufficient biodegradability and accordingly, it does not cause environmental pollution.
On the other hand, uniformity and excellent mechanical strength are required for the wall material of the microcapsules, and the microcapsules are normally prepared by the so-called complex coacervation method.
As the wall material which fulfills the requisites and is easily subjected to microencapsulation, several high molecular substances have been offered, however, at present, gelatin is widely used as the most suitable wall material.
However, in recent years, the utilization of pressure-sensitive recording papers has come to propagate throughout the world and they have come to be used even under a hot and humid climate or very cold climate.
Accordingly, during the period of transportation for the exporting the pressure-sensitive recording papers or the period of warehouse-storage of the papers in the regions of extreme climates, the chance of exposure of the pressure-sensitive recording papers to severe environmental conditions or the chance of usage under such severe environmental conditions has been increased.
The term "severe environmental conditions" herein used means the so-called hot and humid environmental conditions of an ambient temperature of about 40.degree. to 50.degree. C. and of a relative humidity of higher than about 80%, and the cold environmental condition of an ambient temperature of lower than about 0.degree. C.
In the case where the pressure-sensitive recording paper is left for a long time under the hot and humid environmental conditions, the solution of the color former included in the microcapsules of the pressure-sensitive recording paper exudes to outside of the capsule with a result of causing undesirable color-development before the use of the paper and of damaging the paper by contamination to prevent the satisfactory color-development in its proper use time. Such a damage might cause a fatal problem that the so-damaged pressure-sensitive recording paper is no more to be offered to actual use.
On the other hand, in the case where the pressure-sensitive recording paper is used in the very cold region, it takes a long period of time to develop a sufficiently visible color and accordingly, the recording can not be read for a considerable time period and so there is a problem that the paper is no more to be offered in actual use.
However, hitherto, since the pressure-sensitive recording paper has not been propagated to the degree that it is often exposed to the severe environmental conditions or used in the severe environment, it has not been recognized at all that the pressure-sensitive recording paper should have maintained its excellent properties at its specific important requisite even under the severe environmental conditions. Accordingly, there has never been any literature on the pressure-sensitive recording paper which is able to keep the excellent properties even under the severe environmental conditions.
It will be very important to select the solvent of color former in the microcapsules in order to offer a pressure-sensitive recording paper having the excellent properties even in the severe environmental conditions, and a selection of the solvent has been carried out. However, the selection of the solvent which is able to fulfill the requisites (1) to (6) even in the severe environmental conditions is extremely difficult. The difficulty would be due to the contradiction of the properties which should be fulfilled by the solvent in the climatically hot and humid region and the properties which should be fulfilled by the same solvent in the very cold region. However, quite surprisingly it was found that 1-isopropylphenyl-2-phenylethane had a specific property to fulfill the requisites under the severe environmental conditions, that is, both under the climatically hot and humid environmental conditions and under the very cold environmental conditions of lower than 0.degree. C.
It is an object of the present invention to provide a compound as the solvent dissolving a color former for pressure-sensitive recording paper, which the compound is able to fulfill the specific properties requires:
(1) To have high solubility to dissolve the color former,
(2) To have a high velocity of color-development, a high color-density and a high color stability after color-development, when applied in the pressure-sensitive recording paper,
(3) To be stable against light, heat and chemicals,
(4) To be substantially odorless,
In which the properties (1) to (4) are fulfilled even under the hot and humid environmental conditions, and under the very cold environmental conditions,
(5) To be harmless to the human body, and
(6) To have a sufficient biodegradability, and thereby to offer a pressure-sensitive recording paper which is stable and is possibly put to practical use even under the severe environmental conditions.
The other object of the present invention will be elucidated from the following descriptions: